This proposal has two broad goals. One is to explore fully the parameters that govern the adaptation of loudness and to uncover the mechanism underlying its occurrence and, more importantly, its non-occurrence. Hearing is almost unique in that sensory adaptation is so infrequent; a sound heard continuously usually does not appear to get softer over time. A likely functional reason for this failure of adaptation is that if very loud sounds were to become softer with prolonged exposure, they would be less aversive and organisms would be less likely to avoid or flee them. Remaining in their presence, organisms would end up with noise-induced hearing loss. Indeed, such loss may be incurred more readily from exposure to very high frequencies where even intense tones become not only less aversive but even inaudible within 1 or 2 min. In these conditions and in others where loudness adaptation does take place, people differ a great deal in how much they adapt, and these differences may be related to their susceptibility to noise-induced hearing loss. We shall investigate these problems by having subjects, including some hearing-impaired persons, judge the loudness of tones under various conditions at many different frequencies and levels (remaining well within safe limits) over periods of 6 min. We shall also measure possible effects of such exposures on the loudness of sounds that come on shortly afterwards. An understanding of the mechanisms underlying loudness adaptation will aid in learning how loudness is coded in the nervous system. The other broad goal of this proposal is to identify the functions of the olivocochlear bundle in human hearing. This bundle of some 900 nerve fibers sends signals from the brain to the receptors in the inner ear. To determine how these fibers affect hearing, patients who have had their vestibular nerve cut to relieve intractable vertigo, usually caused by Meniere's disease, will be studied. The olivocochlear bundle is located within the vestibular nerve so that it too is cut. We propose to study the effect of the operation on selective auditory attention, on frequency selectivity, and on loudness adaptation. Patients will undergo special auditory tests after the operation and, whenever possible, also before. Learning just what function the olivocochlear nerve serves should greatly advance our understanding not only of the auditory system but of sensory systems in general.